This invention relates to an AC switch device used for switching AC circuit and an AC switch circuit having the AC switch device, which are in particular used for an AC circuit for an inverter mounted on small household electrical appliance.
Recently, inverters have become used more and more for small household electrical appliance. In this current of using inverters, an AC switch circuit as follows for switching AC at high speed has been used, as means for providing an inexpensive and high-reliable system in comparison with a DC brushless type circuit.
FIG. 1 shows a circuit diagram showing a basic constitution of the AC circuit. A current flowing from an AC power supply A101 flows through a load L101 when a switch SW101 is closed, and is cut off when the switch SW101 is opened.
However, if such a circuit is actually used, as shown in FIG. 2, a switch SW102 for making a regenerative current flow when the switch SW101 is opened is provided. Further, a dead time is provided in order to prevent undesired flow of current between switches SW101-SW102, and there required bidirectional Zener diodes Z101 and Z102 for absorbing a surge generated thereby, and a capacitor C101 for absorbing a surge generated due to an impedance of the AC power supply A101.
Further, constituting an AC switch circuit wherein a current can flow in the two directions as shown in FIG. 3 is means which are effective in respect of cost and electric power loss. In this circuit, AC switch devices AS101 and AS102 capable of running a current in the two directions are used as the switch SW101 and SW102. The AC switch devices AS101 and AS102 are made by connecting two MOS field effect transistors (hereinafter referred to as "MOSFET") in series.
However, in the AC switch device shown in FIG. 3, since two MOSFETs are connected in series, the number of the MOSFETs (elements) increases, and wiring resistance also increases. Therefore, in a conventional AC switch circuit, a number of MOSFETs having a low on-state resistance should be prepared, and there is a problem that lowering the price of the device is inhibited.
Further, with respect to gate driving circuits of the TR101 and TR102, since the source voltages are different, floating-state power supplies must be used for them as shown in FIG. 4 and the power supplies must be separated by using transformers, which complicates the circuit.
Therefore, it is generally known as an inexpensive method to use a circuit as shown in FIG. 5 comprising a bootstrap power supply VBS, a control signal generating circuit 101, a level shift circuit 102, drive circuits 103 and 104, diodes D101 and D102, and capacitors C101 and C102. However, in the case of an AC switch device, it is difficult to apply such a circuit since electric potentials of the both ends of the device are not fixed.